After months of scrutinizing photographs of the lunar surface,
scientists have finally found the crash site of a forgotten rocket stage that
struck the far side of the moon in March.
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They still do not know for sure which rocket the wayward debris
originated from. And they are perplexed about why the impact excavated two
craters and not just one.
“It’s cool, because it’s an unexpected outcome,” said Mark
Robinson, a professor of geological sciences at Arizona State University who
serves as the principal investigator for the camera aboard NASA’s Lunar
Reconnaissance Orbiter, which has been photographing the moon since 2009.
“That’s always way more fun than if the prediction of the crater, its depth and
diameter, had been exactly right.”
Robinson reported the discovery in June on the website that
stores images taken by the lunar orbiter.
The rocket crash intrigue started in January when Bill Gray,
developer of Project Pluto, a suite of astronomical software used in
calculating the orbits of asteroids and comets, tracked what looked like the
discarded upper stage of a rocket. He realized it was on a collision course
with the far side of the moon.
The crash was certain, about 7:25am Eastern time on March 4. But
the exact orbit of the object was not known, so there was some uncertainty
about the time and place of the impact.
Gray said the rocket part was the second stage of a SpaceX
Falcon 9 that launched the Deep Space Climate Observatory, or DSCOVR, for the
National Oceanic and Atmospheric Administration in February 2015.
He was wrong
A NASA engineer pointed out that the launch trajectory of DSCOVR
was incompatible with the orbit of the object Gray was tracking. After some
more digging, Gray concluded that the most likely candidate was a Long March 3C
rocket that was launched from China a few months earlier, on October 23, 2014.
Students at the University of Arizona reported that an analysis
of the light reflected from the object found that the mix of wavelengths
matched similar Chinese rockets rather than a Falcon 9.
But a Chinese official denied it was part of a Chinese rocket,
saying that the rocket stage from that mission, which launched the Chang’e-5 T1
spacecraft, had reentered Earth’s atmosphere and burned up.
A composite satellite image
provided by NASA/GSFC/Arizona State University of four previous impact craters
from Apollo missions on the moon’s surface, none of which created a double
crater like the impact from March of 2022 that scientists recently spotted in a
satellite image from the moon’s dark side.
Regardless of what rocket it was part of, the object continued
to follow the spiraling path dictated by gravity. At the predicted time, it
slammed into the far side of the moon within the 563km-wide Hertzsprung Crater,
out of sight of anyone on Earth.
The Lunar Reconnaissance Orbiter was not in a position to watch
the impact, but the hope was that a freshly carved crater would show up in a
photograph that the spacecraft took later.
Gray’s software made one prediction of the impact site. Experts
at NASA’s Jet Propulsion Laboratory calculated a location a few kilometers to
the east, while members of the Massachusetts Institute of Technology’s Lincoln
Laboratory expected that the crash would occur tens of kilometers to the west.
That meant the researchers had to search a swath about 80km long
for a crater a few tens of feet wide, comparing the lunar landscape before and
after the crash to identify recent disturbances.
Robinson said he worried that “it was going to take us a year of
imaging to fill in the box.”
While the Lunar Reconnaissance Orbiter has photographed the vast
majority of the moon several times over the past 13 years, there are spots it
has missed. It turned out that some of the gaps were near the expected crash
site.
Robinson remembered thinking of Murphy’s Law and joking, “I know
exactly where it’s going to hit.”
Because the crash was predicted a month ahead of time, the
mission team was able to fill in most of the gaps.
Then the search started
Usually, a computer program does the comparison, but that works
best if the before-and-after pictures are taken at the same time of day. For
this search, many of the images were taken at different times, and the
difference in shadows confused the algorithm.
With all the false positives, “we just sat down and had several
people manually going through the millions of pixels,” Robinson said.
Alexander Sonke, a senior in Arizona State’s geological sciences
department, contributed to the effort. He estimated that he had spent about 50
hours over several weeks performing the tedious task.
Sonke graduated in May. He got married. He went on his
honeymoon. A few weeks ago was his first day back at work — he is about to
embark on his graduate school studies with Robinson as his adviser — and he
resumed the search for the impact site.
He found it
Sonke said he had seen “a group of pixels that looked
significantly different in brightness” as the before-and-after images blinked
back and forth.
“I was pretty confident when I saw it that this was a new
geologic feature,” Sonke said. “I certainly jumped out of my seat a little, had
a feeling that this was definitely it, and then tried to kind of restrain my
excitement.”
The eastern crater, about 18m in diameter, is superimposed on
the slightly smaller western one, which most likely formed a few thousandths of
a second before the eastern one, Robinson said.
This is not the first time a spacecraft part has hit the moon.
For example, pieces of the Saturn 5 rockets that took astronauts to the moon in
the 1970s also carved craters. But none of those impacts created a double
crater.
The reason this one did might point to its mystery identity. The
October 2014 Chinese mission carried the Chang’e-5 T1 spacecraft, a precursor
for another mission, Chang’e-5, which landed on the moon and brought rock
samples back to Earth.
The precursor T1 spacecraft did not include a lander, but
Robinson surmises that it had a heavy mass at the top of the stage to simulate
the presence of one. If so, then rocket engines at the bottom and the lander
simulator at the top could have created the two craters.
“That’s sheer speculation on my part,” Robinson said.
The other parts of the rocket stage would have been thin, light
aluminum, not likely to make much of a dent on the lunar surface.
The actual impact site lay between the sites predicted by Gray
and the NASA Jet Propulsion Laboratory, close to the NASA one. “It was within
the margins of error that we had computed,” Gray said.
It was also fortunate that the Lunar Reconnaissance Orbiter team
had filled in the gaps — called gores, in the language of mapmakers — in the
images. “As Murphy would have it, that thing impacted in what was one of the
gores,” Robinson said. “If I hadn’t been alerted, we wouldn’t have had a before
image.”
The scientists might eventually have found the crash site. Dirt
tossed out from a gouged crater is usually brighter, growing darker over time.
That is how scientists identified the craters caused by Saturn 5 stages.
But they would still be looking for one small bright spot in the
haystack of the moon.
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